In reproducibility test, use disposable output_base

tests/core/reproducibility/test.sh

@@ -1,18 +1,35 @@
 #!/bin/bash
 set -eu -o pipefail
 
-BAZEL_OPTS=(
-"--bazelrc=/dev/null"  # Disable any user specified remote/disk caching
-)
-
 function run_bazel() {
-    bazel "${BAZEL_OPTS[@]}" clean --expunge
-    bazel "${BAZEL_OPTS[@]}" build //tests/core/go_binary:all //tests/core/cgo:opts_test
+    local output_base
+    local bazel_opts
+    local build_opts
+
+    output_base=$(mktemp -d)
+    bazel_opts=(
+    "--bazelrc=/dev/null"  # Disable any user specified remote/disk caching.
+    "--output_base=${output_base}"  # Use a disposable output_base.
+    )
+    build_opts=()
+
+    if [[ $(uname -s) == "Linux" ]]; then
+      # Use shared memory sandbox for faster runs.
+      build_opts+=("--experimental_sandbox_base=/dev/shm")
+    fi
+
+    bazel "${bazel_opts[@]}" build "${build_opts[@]}" \
+      //tests/core/go_binary:all \
+      //tests/core/cgo:opts_test
+
     {
       find bazel-out/ -name '*.a' | sort | uniq | grep stdlib | xargs shasum
       find bazel-bin/tests/core/go_binary -name 'custom_bin' -exec shasum {} \;
       find bazel-bin/tests/core/cgo -name 'opts_test' -exec shasum {} \;
     } > "$1"
+
+    chmod -R u+w "${output_base}"  # Give write permissions so we can use `rm -rf`.
+    rm -rf "${output_base}"
 }
 
 FILE1=$(mktemp)
@@ -26,6 +43,7 @@ run_bazel "${FILE2}"
 
 echo Diffing runs
 diff "${FILE1}" "${FILE2}"
+echo Builds are identical!
 
 echo Removing files
 rm "${FILE1}" "${FILE2}"